1. Field of the Invention
The present invention relates to a device and method for identifying an event, among events detected in the atrium in a heart, as an atrial depolarization.
2. Description of the Prior Art
An atrial depolarization manifests itself as a P wave when cardiac activity is recorded in an ECG. The corresponding depolarization in the ventricle gives rise to a QRS complex or an R wave in the ECG. In, e.g., physiological studies or in the treatment of the heart with an electrical heart stimulator such as a defibrillator, pacemaker etc., reliable identification of an atrial depolarization among events sensed in the atrium is important in many cases.
Reliable identification of the P wave is a problem, however, as illustrated below with an example from dual-chamber pacing. A dual chamber pacemaker can operate in different modes, usually designated with a three-position alphabetic code in which the first letter indicates stimulation in the atrium (A), ventricle (V) or both (D), the second letter indicates sensing in the atrium (A), ventricle (V) or both (D) and the third letter indicates the pacemaker's operating mode, i.e. triggered (T), inhibited (I) or both (D). For simplicity, these alphabetic designations will be used where appropriate in the description below, the letter A thus generally designating the atrium.
Thus, a dual chamber pacemaker operating in the DDD mode stimulates and senses in both the atrium and the ventricle, and its mode is either inhibited or triggered as needed. In the inhibited mode, the pacemaker's stimulation pulse is suppressed in the case of the atrium when a P wave is sensed, and in the case of the ventricle when a QRS complex or R wave is sensed. In other words, the pacemaker only stimulates if the heart's intrinsic signals are not sensed at the right time.
The correct operation of such a dual chamber pacemaker obviously depends on the ability of the pacemaker's sensing electronics to accurately sense the P wave for the atrium (atrial channel) and the R wave for the ventricle (ventricular channel), respectively. This cannot always be reliably accomplished because of interference, and pacemaker operation can then be affected. Sources of interference may be both inside and outside the heart and can affect sensing in both the atrial channel and the ventricular channel.
Since the present invention is for achieving reliable identification of the P wave, only interference problems in the atrium/atrial channel will be exemplified below.
One problem in the sensing of the P wave is caused by the circumstance that the QRS complex or R wave generated by the ventricle has an amplitude greatly exceeding the amplitude of the P wave. Thus, when crosstalk occurs in the heart, the R wave causes detection of a spurious P wave in the atrium with an amplitude which is equal to or often greater than the amplitude of the true P wave. Here, "crosstalk" means that the R wave is propagated to the atrium by electrical conduction in blood and tissue and sensed there by the pacemaker's sensing electronics for the atrium, i.e. far-field sensing of the R wave. The propagation time for the R wave in this context is on the order of 10 milliseconds. (For clarity, it should be noted that R wave crosstalk should be distinguished from retrograde conduction of the R wave to the atrium. In retrograde conduction, the myocardium's cells depolarize, and propagation time is on the order of 100 milliseconds. Retrograde conduction can also give rise to spurious P waves, but this phenomenon will not be discussed here.) A spurious P wave occurring in crosstalk can cause the pacemaker to mistakenly emit a stimulation pulse in the ventricle at a time corresponding to its repolarization, since the spurious wave occurs at a time corresponding to the R wave and not to atrial depolarization. In repolarization, which causes a T wave in the ECG, the ventricle is sensitive (the vulnerable phase) to electrical stimulation, and a pacemaker pulse delivered to the ventricle at this time could induce tachycardia or, at worst, fibrillation. These conditions are capable of causing cardiac arrest.
As noted above, other sources of interference can cause problems in the sensing of the P wave. Interference generated by, e.g., external electrical equipment can, through far-field sensing, cause spurious P wave sensing. Susceptibility to spurious P wave sensing related to far-field sensing depends on electrode placement in the atrium. The risk of spurious P wave sensing is particularly great if a separate electrode, affixed to the wall of the atrium, is not used for sensing, as can occur in the VDD mode (when the atrium is only sensed, not stimulated), but the electrode cable, whose tip is in the ventricle, is instead provided with electrode surfaces for sensing in the atrium, these electrode surfaces located so that a "floating" electrode results, i.e. the electrode is freely immersed in the blood of the atrium.
The prior art discloses a number of ways to avoid identification of spurious P waves as genuine. A spurious P wave often has, e.g., a different frequency content or appearance (morphology) from a genuine P wave. Filtering or some other form of signal processing have thus been used for discriminating spurious P waves. ECG signals, however, do not have the same morphology in different patients. This is because of the differing size and shape of the heart and/or differing placement of electrodes in different patients. In addition, ECG signals from the same patient may have different morphologies at different times because of, e.g., different transient pathological conditions in her/his heart and/or medication given to treat these conditions. In a heart, VES Ventricular extrasystoles which are fed back to the atrium, also have a morphology which greatly differs from a normal QRS complex. As a result of these variations in ECG signals, properly set sensing electronics, operating with filtration or morphology-processing signal conditioning capable of reliably identifying a genuine P wave on a particular occasion in a particular patient, may be totally incapable of achieving this identification in the same patient, or in another patient, on another occasion.
Another known way to avoid the sensing of spurious P waves is to impose a PVARP (post-ventricular atrial refractory period) with a duration suitable for the atrial channel's sensing electronics, after a spontaneous or stimulated QRS complex is sensed in the ventricle. The PVARP is achieved by blanking, i.e., the atrial channel's electronics are made insensitive by, e.g., cutting off the supply of power thereto. A blanking interval should be selected which is so long that atrial events capable of triggering stimulation in the ventricle during the vulnerable phase cannot be sensed. However, the interval must not be too long, since enough time must be left for the electronics to sense any genuine P wave for inhibition of the pacemaker before the pacemaker, in the DDD mode, triggers an atrial stimulation. The choice of an appropriate or optimum length for the blanking interval is often unfortunately associated with difficulties similar to those in the above described morphological and filtering techniques and therefore depends on both the choice of patient and the individual patient's condition on a particular occasion. In recent years, attempts have been made to resolve the difficulty in finding an optimum interval by introduction of a two-part blanking interval for the electronics, the first part of which an absolute refractory period (initial blanking interval) and the second part a relative refractory period, the blanking interval restarting if a signal is sensed during the relative refractory part of the blanking interval. This technique, in which prolongation of the blanking interval can be achieved up to a certain period of time after blanking is instituted the first time, is described in U.S. Pat. No. 4.974.589.